Somatic gene therapy for man inherited metabolic diseases will require genetic modification of liver cells. We plan to develop methods for liver-directed gene therapy using jaundiced mutant rats (Gunn strain) as a model. Human subjects with Crigler-Najjar syndrome, Type I lack UDP-glucuronosyltransferase (UGT) activity toward bilirubin. Because UGT-mediated glucuronidation of bilirubin is essential for its biliary elimination, patients with Crigler-Najjar syndrome, Type I exhibit life-long hyperbilirubinemia and often die of kernicterus in infancy or childhood. The Gunn rat has a similar metabolic deficiency and is a particularly useful model because (a) the biochemical, pathophysiological and clinical aspects of hyperbilirubinemia are well described and the normal bilirubin-UGT gene has been cloned, (b) phenotypic correction will require gene transfer into the liver because liver is the primary organ that extracts bilirubin from plasma and conjugates and excretes it in bile and (c) no effective long-term therapy for Crigler-Najjar syndrome, Type I is available other than liver transplantation. The Gunn rat will be used to develop two different types of gene replacement therapies. In the ex vivo approach, hepatocytes will be harvested from one or more liver lobes of a Gunn rat, a functional bilirubin-UGT gene will be introduced into the hepatocytes and the genetically modified cells will be transplanted back into the donor Gunn rat. Because the method involves autologous transplantation, immune rejection is not a problem and immunosuppression will not be needed. Another approach, which is potentially less morbid, is endocytosis-mediated targeted delivery of B-UGT genes to hepatocytes in vivo. The gene targeting method will be based on the interaction of a carrier asialoglycoprotein with a hepatocyte-specific receptor. Stable expression of the targeted gene will be achieved by developing vector systems that may persist as extrachromosomal elements or translocate to the nucleus and integrate with the host genome, and by appropriate manipulations of the recipient. The development of liver-directed gene therapies in the Gunn rat should greatly facilitate subsequent application of these methods to the treatment of Crigler-Najjar syndrome, Type I and other inherited metabolic diseases of the liver.